Membrane structure for electrochemical sensor

ABSTRACT

A micro-electrochemical sensor contains magnetic compounds inserted within a substrate that exert a magnetic force of attraction on paramagnetic beads held in contact with an electrode. The magnetic compounds can be contained within a fluid that is introduced into a void in the substrate. The electrode can be spaced apart from the magnetic compounds by a dielectric multi-layer membrane. During the fabrication process, different layers within the membrane-electrode structure can be tuned to have compressive or tensile stress so as to maintain structural integrity of the membrane, which is thin compared with the size of the void beneath it. During a process of forming the structure of the sensor, the tensile stress in a TiW adhesion layer can be adjusted to offset a composite net compressive stress associated with the dielectric layers of the membrane. The membrane can also be used in forming both the electrode and the void.

BACKGROUND

1. Technical Field

The present disclosure relates to the fabrication ofmicro-electrochemical devices.

2. Description of the Related Art

A micro-electrochemical sensor can be formed, in a substrate, as amicroscopic structure that provides a platform for a chemical process orreaction. Some reactions cause electrical effects such as changes involtage or current that can be sensed by electrodes attached to themicrostructure. Thus, such a microstructure provides an electricaldetector that can be used to monitor the chemical process. Commonexamples of micro-electrochemical sensors include biosensors such as,for example, immunosensors that can be used to analyze biologicalsamples, as described in U.S. Patent Application Publication No.2012/0034684 (hereinafter, “the '684 patent application”).

Semiconductor microstructures having dimensions in the range of about1-10 microns can be manufactured for biotechnology applications usingtechniques, materials, and equipment similar to those that have beendeveloped for the microelectronics industry. For reliability, it isimportant that microstructures that make up the platform aremechanically stable. In general, achieving a mechanically stablemicrostructure can be particularly challenging if, for example, thedimensions or the material properties of adjacent microstructuralelements are very different from one another. For example, a multi-layermicroscopic structure situated next to a macroscopic structure can bevulnerable to destructive events such as cracking, rupturing, peeling,or delamination of the layers. Such events can occur in response todevelopment of a composite shear stress that results from imbalances incompressive and tensile stresses associated with the various layers ofthe microstructure. Structural instabilities that cause peeling ordelamination of thin films on a silicon substrate are a recurringproblem in the fabrication of electronic devices. Historically, this hasbeen especially problematic near the boundaries of “trench isolation”areas formed in the silicon as electrical boundaries between neighboringtransistors.

BRIEF SUMMARY

Different layers within a multi-layer membrane on a silicon substratecan be tuned to have compressive or tensile stress so as to maintainstructural integrity of the membrane, during and after formation of alarge void in the substrate beneath it. In particular, a TitaniumTungsten (TiW) adhesion layer having a selectable amount of tensilestress is found to be beneficial in maintaining secure contact betweenthe electrode portion of the membrane and the dielectric portion of themembrane.

During a process of forming the structure of the multi-layer membrane,the tensile stress in the TiW layer can be adjusted to offset acomposite net compressive stress associated with the dielectric layersof the membrane. Furthermore, the membrane dielectric can be used as anetch stop during formation of the electrode on a front side of thesubstrate, and also during formation of the void on a back side of thesubstrate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements.The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale.

FIG. 1 is a side view of an exemplary micro-electrochemical sensordesign in which magnetic beads used as sensors detect magnetic compoundsthrough a multi-layer membrane.

FIG. 2 is a high level process flow diagram illustrating the order offront side and back side processing steps for producing the structure ofFIG. 1.

FIG. 3 illustrates components of the multi-layer membrane shown in FIG.1.

FIG. 4 is a detailed process flow diagram showing a sequence of processsteps that can be used to produce the membrane portion of the structureshown in FIG. 1.

FIG. 5 is a side view of a membrane design indicating relativethicknesses and stresses of each layer.

FIGS. 6-9 show a sequence of side views of increasing magnification,wherein the images are derived from cross-sectional scanning electronmicroscope (SEM) images of actual samples that maintained structuralintegrity throughout the fabrication process.

FIG. 10 is a bottom plan view derived from an optical image of afinished membrane-electrode structure as seen from the back side of thesubstrate.

DETAILED DESCRIPTION

It will be appreciated that, although specific embodiments of thepresent disclosure are described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the present disclosure. Accordingly, the present disclosure isnot limited except as by the appended claims.

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various aspects of thedisclosed subject matter. However, the disclosed subject matter may bepracticed without these specific details. In some instances, well-knownstructures and methods of semiconductor processing comprisingembodiments of the subject matter disclosed herein have not beendescribed in detail to avoid obscuring the descriptions of other aspectsof the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thespecification are not necessarily all referring to the same aspect.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more aspects of the presentdisclosure.

Reference throughout the specification to a dielectric multi-layermembrane can include membranes other than those used to illustratespecific embodiments of the sensor device presented. The term “membrane”should not be construed narrowly to limit a micro-electrochemicalstructure solely to a three-layer membrane on a silicon substrate, forexample, but rather, the term “membrane” is broadly construed to cover adielectric that provides spacing between an electrode and one or moremagnetic compounds inserted into a substrate.

Specific embodiments are described herein with reference to examples ofmicro-electrochemical sensor structures that have been produced;however, the present disclosure and the reference to certain materials,dimensions, and the details and ordering of processing steps areexemplary and should not be limited to those shown.

In the figures, identical reference numbers identify similar features orelements. The sizes and relative positions of the features in thefigures are not necessarily drawn to scale.

With reference to FIGS. 1 and 2, an exemplary micro-electrochemicalsensor 100 is shown, in which a plurality of paramagnetic beads 102 areplaced in contact with an electrode 104. The electrode 104 is locatedabove, and spaced apart from, a substrate 106 by a supporting means suchas a dielectric layer 107. The electrode 104 and the dielectric layer107, together, form a multi-layer membrane 108. The substrate 106 has afront side 109 and a back side 110, onto which films may be deposited,or to which residual films may adhere. Once the membrane 108 is inplace, the front side 109 becomes a “membrane-substrate interface.” Oneor more voids 111, formed in the substrate 106, can accept a sample of amaterial that contains one or more magnetic compounds 112.

Paramagnetic materials are defined by their intrinsic magneticpermeability being greater than one. This property causes a paramagneticmaterial to respond to externally applied magnetic fields, withoutretaining any magnetization after the field is removed. Examples ofparamagnetic materials include, for example, magnesium, molybdenum, andtantalum. In the exemplary sensor 100, the beads 102 have asubstantially spherical shape, so that they are free to roll about onthe surface of the electrode 104, unless they are held in place on theelectrode by a force of magnetic attraction to the magnetic compounds112 placed in the substrate 106. Variations in the magnetic forcebetween the magnetic compounds 112 and the beads 102 result invariations in electric currents within the electrode 104. These varyingelectric currents can be sensed by an external circuit connected to theelectrode 104. If the void 111 contains no sample, or if it contains asample that lacks enough magnetic compounds, there may be aninsufficient force to maintain the position of the beads, and thissituation produces a measurable fluctuation in the electric current.Thus, the device functions as a detector for the presence of, and theamount of, the magnetic compounds 112 within the void 111.

The paramagnetic beads 102 can be made of any solid paramagneticmaterial. According to the embodiments described, the beads 102 can havea diameter ranging from about 1-50 microns.

The magnetic compounds 112 can be components of a material, such as abiological material, that has a magnetic signature, and which is capableof being introduced into the void 111. One application of the sensor 100is a detector that senses the presence of the magnetic compounds 112within a blood sample. In this embodiment, the magnetic compounds 112can be blood cells that are magnetized, or that contain magneticallysensitive elements, for example, due to the presence of iron. However,embodiments of the invention are not so limited. The magnetic compoundscan be inserted into the void via other types of carrier materials suchas, for example, a gel, a paste, a powder, or a fluid, or without acarrier material. In one embodiment of the sensor 100, the dimensions ofthe void are much larger than the thickness of the multi-layer membrane108. In other embodiments of the device 100, a liquid sample can spreadout into interstitial regions (e.g., small voids) within the substrateinstead of remaining localized in a large void.

The electrode 104 can be made of any electrically conducting material,such as a metal, metal alloy, and combinations thereof. The electrode104 can include multiple layers, collectively referred to as a “metalstack.” For bio-compatibility reasons, electrodes that include one ormore layers made from noble or non-reactive metals such as gold, can beadvantageous if the sensor is deployed within a biological system, oranother type of aqueous or oxygen-rich environment. Such noble metalsare corrosion resistant, and generally provide higher quality electricalsignals.

The substrate 106 can be made of a semiconductor material such assilicon that benefits from a variety of existing processingtechnologies, but any substrate material that can be used as a platformon which the membrane 108 can be formed or attached, would be a suitablealternative.

Analysis of biological samples using existing magneticmicro-electrochemical sensors are described in more detail inpublications such as the '684 patent application mentioned above, aswell as in U.S. Pat. Nos. 7,419,821, 7,682,833, and 7,723,099, amongothers.

The disclosure herein is concerned particularly with the structure andformation of the membrane 108 on the front side 109 of the siliconsubstrate 106, and a large void 111 formed through the back side 110 ofthe substrate 106. A large void is one which is about 10-100 times widerthan the thickness of the membrane. The membrane layers on the frontside 109 are designed to help maintain structural integrity of thedevice 100, when it is situated over a large void 111, however, theyalso have a second purpose. According to an exemplary embodiment, thethin films making up the dielectric layer 107 are also deposited on theback side 110 so that, when the back side 110 is patterned, a back sidelayer serves as a mask for removing substrate material to create thevoid 111.

With reference to FIG. 2, a high-level process flow 200 is presented todescribe formation of the structures shown in FIG. 1. Detaileddescriptions of the process steps 202, 204, 206, 208, 210, 212 arepresented below. Because the device 100 is formed by processing bothsides of the substrate 106 at various times, a preliminary high-leveldiscussion offers some additional clarification. In steps 202 and 204,oxide and nitride layers are formed on both sides of the substrate 106.On the front side 109, these layers form the dielectric layer 107 of themembrane 108, which remains a blanket, unpatterned structure throughoutthe process, and remains in the finished device 100. On the back side110, these layers are patterned in step 206 and used as a hard mask 207to permit formation of the deep void 111. When the hard mask patterningprocess is complete, a two-layer nitride/oxide hard mask has beenformed, exposing regions of the underlying silicon substrate 106. Next,the front side is treated to deposit and pattern the metal electrode 104in steps 208 and 210, respectively. These steps can occur in plasmadeposition and etching equipment in which only the front side 109 of thesubstrate 106 is exposed to processing. After the electrode 104 isformed on top of the blanket dielectric layer 107, the back side 110 isfurther processed to selectively etch the deep void 111. This can bedone by immersing the sample in a wet chemical etch that consumesexposed areas of the silicon substrate 106 at a high rate, but will notattack either the dielectric layer 107, or the hard mask 207. Thus, thesilicon etch stops at the membrane-substrate interface at the front side109.

The layered structure 300 of the device 100 is shown in greater detailin FIG. 3, and with reference to corresponding detailed process stepsshown in FIG. 4 as a process flow 400. The membrane 108 can includemultiple constituent metal films within the electrode 104 and multipleconstituent dielectric films within the dielectric layer 107. In thepresent embodiment that utilizes a silicon substrate 106, componentfilms within the dielectric layer 107 can include, for example, two ormore dielectric materials such as a silicon oxide layer 302, siliconnitride films 304 and 306, and a silicon carbide film 308. The siliconoxide layer 302 can be, for example, SiO₂ that is grown by a thermaloxidation process 402. Thermal oxidation of the bare silicon substrate106 occurs when the substrate is exposed to an oxygenated environment ina hot furnace. If a batch process is used in which the substrate can beheld on its edge, an oxide layer will form on both the front side 109and the back side 110 of the silicon substrate 106. According to oneembodiment, the thermal SiO₂ layer 302 is about 1.3 to 1.6 micronsthick. In one alternative embodiment, a TEOS or other high density oxideis then deposited onto layer 302.

Next, a silicon nitride film 304 can be deposited by a low-pressurechemical vapor deposition (LPCVD) process 404 that is also carried outinside a furnace, with both sides of the substrate exposed, such thatthe nitride deposition also occurs on both sides of the substrate 106.According to one design embodiment, the LPCVD silicon nitride film 304adjacent to the SiO₂layer 302 is about 0.3 microns thick.

When deposition of the multi-layer membrane 108 is complete, the oxideand nitride layers 302 and 304, respectively, that have been formed onthe back side 110 of the substrate 106, can be patterned together usinga conventional photolithography process. First, a photolithography backside coating process can be used to mask the nitride surface of the backside 110 with photoresist. The photoresist is exposed through a reticleand developed to form a back side mask used to pattern exposed areas ofthe nitride film for etching. In the present embodiment, a dry etchprocess 408 is used to etch the back side nitride film 304, and the dryetch process 408 is then followed directly by a wet etch process 410 toremove corresponding areas of the oxide film, using the patternednitride film as a mask for the oxide. The etch chemistry used for thewet etch process 410 is selective to not etch nitride. When the patternhas been etched into both the nitride and oxide films, the photoresistcan be stripped in a process step 412, leaving behind a multi-layerdielectric mask. In other embodiments, different etching processes canbe used, and in general, an alternative lithography process ornon-lithography technologies can be used to pattern the back sidedielectric layers.

Next, a second silicon nitride film 306 can be deposited by aplasma-enhanced chemical vapor deposition (PECVD) process step 414 thatis carried out inside a vacuum chamber such that the second nitride film306 only forms on the front side 109 of the silicon substrate. Thesecond nitride film 306 is thus adjacent to the un-patterned front sideLPCVD silicon nitride film 304. According to the present embodiment, thePECVD silicon nitride film 306 is within the range of about 0.5-2.0microns thick.

Next, an optional silicon carbide layer can be deposited in aplasma-enhanced chemical vapor deposition (PECVD) process step 414 thatis carried out inside a vacuum chamber, thus only contributing materialto the front side of the substrate 106, on top of the second nitridefilm 306. According to this alternative embodiment, the thickness of theoptional PECVD silicon carbide film 308 is within the range of about0.125-0.25 microns. In some embodiments, the silicon carbide layer 308is not present.

The electrode 104 is then formed adjacent to the multi-layer membrane108. The electrode 104 can be formed by depositing a metal stackcomprising two or more metal layers and then patterning the two layerstogether. For example, an adhesion layer 310 can be deposited first, asa means of securing the electrode by reducing the likelihood ofdelamination of the electrode 104 from the membrane 108. The adhesionlayer is followed by a bulk metal layer 312. According to an exemplarydesign, the adhesion layer 310 can be a 0.025-micron thick layer of aTitanium Tungsten (TiW) alloy, and the bulk metal can be approximately0.1125 microns of gold. Both metal layers can be deposited by asputtering process 416 inside a vacuum chamber so that only the frontside of the substrate 106 receives metal. An electroplating processwould not be a suitable alternative for metal deposition for the presentdevice because immersion in an electroplating solution would depositmetal onto the back side 110.

After metal deposition, the electrode 104 can be patterned using astandard lithographic process that entails a photoresist masking step418, a metal etching step 420, and a photoresist stripping step 422. Themetal etching step 420 can be either a dry etch, a wet chemical etch, ora sequence of both wet and dry etch steps. The membrane 108 is designedto serve as an etch stop layer for the metal etching step 420. Thereforethe etch chemical used in etching step 420 is chosen to have a highselectivity to the dielectric films used in the membrane. According tothe present method, the resulting metal pattern that defines the size ofthe electrode has a critical dimension, or a selected width, that ischosen so as to increase the likelihood of achieving a stablemicrostructure.

Finally, the void 111 is formed in the back side 110 of the substrate106 by etching a deep, graded trench in the silicon using a solution oftetramethyl ammonium hydroxide (TMAH) in step 424. The substrate etchuses the multi-layer dielectric mask formed in process steps 406, 408,410, and 412. The TMAH is necessarily selective to both nitride andoxide to preserve the multi-layer mask. After exposing and developingthe photoresist, a deep trench is etched starting at the back side 110and continuing through the silicon until it reaches themembrane-substrate interface at the front side 109, which serves as anetch stop for the back side silicon etch. If an etch chemistry otherthan TMAH is used to form the void 111, it has a selectivity of the etchchemistry to the materials used in the dielectric membrane that issufficiently high, because the membrane materials are used as both amask layer and an etch stop layer, while the etch rate of the substratematerial must be high to consume a large amount of the substrate. Anadvantage of the multi-layer membrane 108 is its ability to maintainstructural integrity while a large amount of the silicon substrate isbeing removed on one side to create the void 111. Trials using a simpleoxide film instead of the multi-layer approach were not reliable inpreventing structural failure. In particular, a low stress PECVD nitridefilm helps to prevent bulging of the membrane 108 after formation of thevoid 111. An additional advantage of the multi-layer membrane is that itprotects the front side 109 of the substrate from defects induced by theback side coating process. According to an exemplary embodiment, theresulting void 111 measures approximately 525 microns deep and more than100 microns wide at its widest point. The void 111 is then available forinsertion of the magnetic compounds 112.

As the layers of material in the membrane 108 and the electrode 104 areformed, it is possible to control the mechanical stress in each layer ofmaterial by varying process equipment parameters such as temperaturesand pressures within the processing chamber during each deposition step.Tuning the mechanical stresses in this way can result in an overallcomposite stress characterizing the membrane and electrode structure asa whole that is only slightly compressive, but not prone todelamination. If the stresses are not tuned, delamination can occur fromthe front side 109 or at the interface between the electrode 104 and thedielectric layer 107, either at the time when the void 111 is formed, orthereafter. During deposition of the adhesion layer, in particular,equipment parameters can be adjusted so that the resulting adhesionlayer 310 is configured to exhibit a selectable amount of tensile stressin order to offset the composite compressive stress present in themembrane. Because the process parameters are often equipment-dependent,determining the proper adjustments can be done more effectively byrepeated experimentation, using film stress measurements as a metric ofsuccess.

Thus, it was determined empirically that, for the present embodiment, astable structure like the one shown in FIGS. 6-9 is likely to result ifthe adhesion layer 310 is tuned to a selected tensile stress, and theother layers are tuned to have compressive stress. FIG. 5 illustratesthe relative thicknesses of the different compressive and tensile thinfilms 502, 504, 506, 510, and 512 that form a stress-tuned membrane 500.The stress-tuned membrane 500 is one embodiment of the structure shownin FIG. 2. These films include, in order of deposition, a multi-layermembrane comprising an oxide film 502 having compressive stress in therange of 3.0-5.0 E7, an LPCVD silicon nitride film 504 havingcompressive stress of 5.47 E7, and a PECVD nitride film 506 havingcompressive stress of 4.6 E8; a metal electrode comprising a TiWadhesion film 510 having a tensile stress of 1.01 E10, and a gold film512 having compressive stress of 3.83 E7. Although the stress-tunedmembrane 500 is discussed herein in the context of providing a stablestructure within the device 100, there may be other uses of thestress-tuned membrane 500 to provide stability to other types ofstructures that would otherwise be prone to stress-induced damage.

FIGS. 6-9 are derived from actual SEM images of sample structuresproduced by the process flow 400 illustrated in FIG. 4, after completionof the metallization process. FIGS. 6-9 show, to scale, the differentfilm thicknesses achieved as well as a comparative view of the relativedimensions of the various structural elements within the multi-layermembrane 108, at successively higher magnifications. FIG. 6 is a sideelevation view of a whole, exemplary, fabricated micro-electrochemicaldevice structure 600, shown at 120× magnification. The device structure600 is thus a scaled embodiment of the structure for the sensor 100,prior to introduction of the magnetic sources and detectors. The devicestructure 600 includes the silicon substrate 106 and a multi-layermembrane 108, positioned over a void 111 in the substrate. Amembrane-substrate interface is at the front side 109 of the substrate106. FIG. 6 shows the dimensions of the multi-layer membrane 108relative to the void 111, and demonstrates that the multi-layer membrane108 remains intact despite the extent of the void 111 in the supportingsubstrate 106 underneath the multi-layer membrane 108. The criticaldimension of the multi-layer membrane 108 is the selected top width 610of the void 111 at the membrane-substrate interface, which, for thepresent design, generally exceeds 100 microns. In FIG. 6, for example,the top width 610 measures 124.5 microns. The base width 612 of the void111 opposite the membrane measures about 700 microns and the depth 614of the void 111 (i.e., the thickness of the silicon substrate) is about600 microns.

FIG. 7 is a detail view of a region of interest 700 within FIG. 5, atthe membrane-substrate interface. The magnification used to obtain theimage from which FIG. 7 is derived was 600×. The total measuredthickness 702 of the multi-layer membrane 108 at this magnification isabout 3.6 microns. The total thickness of the dielectric layer 107 canbe matched to the radius and material of the beads by design so thatmagnetic forces, which decrease with distance across the dielectric, arestrong enough to be detected by the paramagnetic beads.

FIG. 8 is a membrane-electrode detail view 800 derived from a scanningelectron micrograph at 20,000× magnification, showing individual filmsformed within the multi-layer membrane 108, and indicating theirrelative dimensions. The embodiment shown omits the optional siliconcarbide film. The films shown are, from bottom to top, the silicon oxidelayer 302, the LPCVD silicon nitride film 304, the PECVD silicon nitridefilm 306, and the electrode 104. In the embodiment shown in FIG. 8, thethickness 802 of the oxide 302 measures 1.28 microns, the thickness 804of the LPCVD silicon nitride 304 measures 0.29 microns, the thickness806 of the PECVD silicon nitride 306 measures 1.90 microns, and thethickness 808 of the metal stack measures 119.3 nm.

FIG. 9 is an electrode detail view 900 of the type derived from ascanning electron micrograph of the electrode 104. The electrode 104 isin contact with the PECVD nitride film 306. In the embodiment shown, theelectrode 104 includes the TiW adhesion layer 310 measuring 22.0 nm, anda bulk metal layer 312 of gold measuring 93.7 nm. The TiW adhesion layerreduces the overall stress within the multi-layer membrane 108 and alsoassists in adhering the bulk metal layer 312 to the nitride film 306.

FIG. 10 is derived from an optical image of a fabricatedmembrane-electrode structure 1000, as taken from the back side 110through the substrate 106. The structure 1000 includes a dielectriclayer 107 in contact with a patterned electrode 104. The patternedelectrode 104 is complete after the photoresist stripping step 422. Inthe embodiment shown in FIG. 10, a length 1010 of the patternedelectrode 1008 measures about 700 microns.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. An apparatus that includes a multi-layermembrane on a silicon substrate, the apparatus comprising: a dielectriclayer that extends across a cavity between portions of the siliconsubstrate, the dielectric layer having a first compressive stress; anadhesion layer in contact with the dielectric layer, the adhesion layerhaving a tensile stress; and a metal electrode in contact with theadhesion layer, the metal electrode having a second compressive stressand a width.
 2. The apparatus according to claim 1, wherein thedielectric layer includes two different dielectric materials.
 3. Theapparatus according to claim 2, further including a silicon carbide filmthat is one of the dielectric materials.
 4. The apparatus according toclaim 2, wherein the dielectric materials include silicon dioxide andsilicon nitride.
 5. The apparatus according to claim 1, wherein thedielectric layer is above the cavity in the silicon substrate.
 6. Theapparatus according to claim 5, wherein a ratio of a width of the cavityto the width of the metal electrode exceeds two.
 7. The apparatusaccording to claim 5, having a membrane thickness, wherein the ratio ofthe width of the cavity to the membrane thickness exceeds
 10. 8. Theapparatus according to claim 5, further comprising magnetic compoundscontained within the cavity.
 9. The apparatus according to claim 1,wherein the metal electrode includes gold.
 10. The apparatus accordingto claim 1, wherein the adhesion layer is made from a tungsten alloy.11. The apparatus according to claim 8, wherein a fluid occupies thecavity.
 12. The apparatus according to claim 8, further comprising aplurality of paramagnetic beads, wherein the metal electrode is capableof carrying an electric current induced by a magnetic force ofattraction between the magnetic compounds and the paramagnetic beads,each paramagnetic bead held in contact with a top surface of the metalelectrode by the magnetic force.
 13. The apparatus according to claim12, wherein an induced electric current carried by the metal electrodeindicates an amount of the magnetic compounds.